You can find out about cycling here - A Beginners Guide to the Freshwater Aquarium Cycle
You need to need a big water change of at least 50-60% every day until the Nitrites levels drop off. The damage though of the fish already being exposed to Ammonia and Nitrite in the uncycled tank has unfortunately already shortened their life span.
The likely reason that the yellow lab hides most of the time is the water parameters required for the other fish are stressing the lab, labs need hard water whereas the others require medium/soft to acidic water.
If you have an airstone, add it to the tank, this will provide extra oxygen that will be beneficial to the fish during this stressful time. It is nearly impossible to say whether the other fish will succumb or not. Doing the DAILY water changes will really help though.
Your tank is very borderline for keeping a fully grown JD, they really need a 75g tank as they get big and are extremely aggressive if they want to be.
You also should get another filter, these fish are messy eaters and it will also help with keeping the water clear and provide more area for beneficial bacteria to grow.
There is a product as well which would be beneficial to add to the filter or if there is not enough room, directly in the tank in a mesh bag. Seachem Purigen as described here
, this will help until the tank cycles fully. It can also be refreshed when it gets used up, so you buy it once and it will last a long time.
I hope you manage to save the other fish, please do NOT replace any fish that do die until the tank is fully cycled, it will only put them at risk.